51st U.S. Rock Mechanics/Geomechanics Symposium,
San Francisco, California, USA
2017. American Rock Mechanics Association
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ABSTRACT: The understanding of geothermal fields requires coupling between heat flow through fractures and induced seismicity. In this context, we develop a semi-analytical approach for estimating (1) thermo-poro-elastic stresses in a fractured geothermal system, and (2) seismicity rates based on the model of Dieterich (1994). Cold water is injected at constant rate into a single fracture surrounded by hot impermeable layers. Heat flow is dominated by advection and conduction respectively inside and outside the fracture. Poro- and thermo-elastic stresses are estimated separately following the nucleus-of-strain concept; and for any potential fault around the single fracture the induced Coulomb stress rates are resolved. For this particular scenario, the thermal stresses are dominant. We show that thermal-stressing rates can induce an increase in the rate of seismicity of more than hundredfold at distance up to 200m from the single fracture. Our fast forward model is suitable for data assimilation, and by predicting the spatio-temporal evolution of the locations of increase/decrease of seismicity rate one might optimize geothermal systems while keeping seismicity at a relatively low magnitude.
The sustainability of geothermal fields is based on a paradox. On one side fractures are targeted for heat-flow improvement, and on the other side these same fractures are avoided because of induced-seismicity risk. More specifically, slip along fractures can enhance the heat flow but can also lead to large seismic events. The grail is to take advantage of fracture conductivity while keeping seismicity at a relatively low magnitude avoiding risks of large earthquakes (Zang et al., 2013). Our present model contributes to a better understanding of this complex coupling between heat transfer, flow, fractures, and earthquakes.
Flow through a fractured geothermal system is often modelled with complex numerical approaches, taking into account the interaction between multiple fractures (Taron et al., 2009; Izadi and Elsworth, 2010; McClure and Horne, 2010). These models give detailed pictures of the Thermo-Hydro-Mechanical couplings, but the drawback of this time-consuming model is that processes and physical parameters cannot be updated via a data assimilation scheme.
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